This invention relates generally to powered pushing units for moving loads from one point to another along a selected path, and more particularly, to powered pushing units for moving loads in one direction while transferring back and forth along a selected path.
Powered pushing units are used in many applications to transfer loads between two or more different points along a predetermined path. For example, in railroad applications pushing units called dog carriages or table sleds move railway cars from one point to another for various purposes. Pusher conveyors use powered pushing units, sometimes called shuttles, to transfer loads from one point to another along the lengths of the conveyors. Some pusher conveyors incorporate a centrally located, stationary motor for providing power throughout the conveyor system. Loads are engaged and transferred with one or more pushing elements, known as a dog or dogs, that are located apart from the stationary motor. The motor then supplies power to the dogs remotely with the use of a separate power track or with the use of a roller and pulley or a drive chain arrangement. The dogs may either contact the load directly or contact one or more load bearing trolleys or carriages supported from a second driving track. Some designs incorporate toggle or pivot mechanisms that allow for variable or one-directional engagement of loads with a dog.
For example, U.S. Patents to Hoehn (U.S. Pat. No. 4,072,111) and Curry, et al. (U.S. Pat. No. 3,451,352) each disclose multiple-track conveyor systems where two or more independent, closed loop driving tracks include a plurality of pushing dogs that independently engage one or more load-bearing trolleys. The trolleys themselves have power engaging dogs along a separate load-bearing track. Mechanisms variably disengage the pushing dogs by retracting power engaging dogs when contact is made with adjacent trolleys. In their construction, these systems require room and added expense for at least three separate tracks and many individual dogs, along with additional maintenance time and costs for a large number of components. Since each driving track is a closed loop, breakage of any component of the closed loop necessarily making the entire driving track inoperative. Furthermore, since each driving track is also solely responsible for transferring loads along a particular leg of the conveyor, inoperativeness of any one driving track normally leads to inoperativeness of the entire conveyor system.
Some of the inherent shortcomings of these closed loop systems can be overcome with the use of unitary, individually powered pushing units. Such units typically incorporate a toggle or pivot mechanism for actively engaging or for bypassing a load along the length of the conveyor track. U.S. Patents to Saxonmeyer (U.S. Pat. No. 3,556,011) and Hunt (U.S. Pat. No. 3,522,772) each disclose railroad car movers having pushing units capable oftwo-directional travel and incorporating toggle mechanisms for allowing dogs to rigidly engage loads in a first direction and for bypassing loads in a second direction. U.S. Patent to Morikiyo (5,695,044) discloses a pushing unit allowing for the same one-directional engagement but incorporating a more simple dog and pivot mechanism. Power for the pushing unit is provided externally, such as by the use of a chain or pulley. In all of the foregoing pushing unit designs, forward load engagement in the first direction is rigid, and no mechanism for releasing excessive loads is disclosed. The lack of such a mechanism may present a problem relating to the dependability and service life of a car moving or other conveyor system. For example, in the case where a conveyor is used to transfer variably loaded pallets, accidental overloading of one or more pallets can result in a pushing unit encountering a force greater than the pushing unit""s intended transfer capacity. Such an excessive load force can result from one or more loads becoming jammed along the path of the pushing unit. In the absence of a suitable release mechanism, excessive wear or damage to the pushing unit or power source may also result. Although a suitable trip mechanism may be incorporated into a system to terminate power to the pushing unit upon the occurrence of such an overload, such a solution normally requires a temporary shutdown of the entire system and for some systems may lead to the need for further repairs or maintenance of the conveyor. Where multiple pushing units in a system depend upon a single, centralized power source, other pushing units, as a consequence, remain inoperative during the period of temporary shutdown.
One previous attempt to incorporate an overload release mechanism into a conveyor system does not deal with these shortcomings. U.S. Patent to Janzen, et al. (U.S. Pat. No. 5,437,231) discloses a two-track, continuous conveyor system, the upper track having powered claws pivotally mounted for engaging load-bearing carriages along the second track. The claws are positioned with a simple spring loaded pivot to engage the load bearing carriages and can bypass a carriage when a carriage is jammed or overloaded on the second track. After bypassing a jammed carriage, the claws are incapable of returning to their load engaging position without manual resetting. Since Janzen discloses a continuous conveyor, such resetting necessarily requires a shutdown of the entire system before the claws are again able to engage loads. In addition, Janzen requires additional space and the added expenses that are associated with multiple track systems.
The present invention is a powered pushing unit for transferring loads no greater than a predetermined magnitude in one direction along the length of a load transfer system. The pushing unit includes a frame and a source of power for moving the pushing unit back and forth along the length of the load transfer system.
A dog for engaging a load is pivotally mounted with respect to the frame. A restricting mechanism applies a load engaging force to hold the dog at a load transfer range of positions until the load is greater than the predetermined magnitude. A dog spring which has a dog spring force pivots the dog toward a load engaging position on the dog support arm to engage and transfer loads when the frame is traveling in a first direction. The dog pivots against the dog spring force to bypass loads when the frame is traveling in a direction opposite that in which it pushes loads.
The pushing unit further includes a toggle joint connected between the dog and the frame. The toggle joint forms an angle in a first angular direction to lock the dog in a load engaging position during engagement of a load under the predetermined magnitude. The toggle joint further shifts to form an angle in a second angular direction upon engagement of a load in excess of the predetermined magnitude, thereby allowing the dog to pivot away from the load which is in excess of the predetermined magnitude.
A toggle force mechanism is interconnected between the frame and the pivot of the toggle joint to apply a toggle force, which is substantially less than the predetermined magnitude, to bias the toggle joint in the first angular direction with respect to the pivot. When the dog is pivoted away from the load due to a load in excess of the predetermined magnitude, the toggle force mechanism exerts a force substantially less than the predetermined magnitude on the dog and from the dog to the load. The toggle force mechanism then returns the toggle joint to the first angular direction after the dog passes the load.
Various features, advantages, and characteristics of the present invention will become apparent to one of ordinary skill in the art while reading the following specification. This invention does not reside in any one of the features of the powered pusher unit disclosed above and in the following Detailed Description of Preferred Embodiments and claimed below. Rather, this invention is distinguished from the prior art by its particular combination of features which are disclosed. Important features of this invention have been described below and shown in the drawings to illustrate the best mode contemplated to date for carrying out this invention.
Those skilled in the art will realize that this invention is capable of embodiments which are different from those shown and described below and that the details of the structure of this powered pusher unit can be changed in various manners without departing from the scope of this invention. Accordingly, the drawings and description below are to be regarded as illustrative in nature and are not to restrict the scope of the invention. The claims are to be regarded as including such equivalent powered pusher units as do not depart from the spirit and scope of the invention.